Electron beam system



R. KOMPFNER ET AL 2,857,548

Oct. 21, 1958 ELECTRON BEAM SYSTEM 4 Sheets-Sheet 1 Filed June 10, 1955 'lIIIII/k "'llllll R. KOMPFNER mvewrons W H YOCOM Oct. 21, 1958 R. KOMPFNER ETA]. 2,857,548

- ELECTRON BEAM SYSTEM Filed June 10. 1955 4 sheetshee 2 H. H. YOCOM ,WEWOPS R. KOMPF/VER B MW TW L ATTORNEY Oct. 21, 1958 I KOMPFNER AL 2,857,548

ELECTRON BEAM SYSTEM 4 Sheets-Sheet 3 Filed June 10, 1955 kb1$b0y INVENT R. KR

W H YOCOM BY 4%;

AT TO/PNE V Oct. 21,- 1958 R. KOMPFNER ETAL 2,85

ELECTRON BEAM SYSTEM 4 Sheets-Sheet 4 Filed June 10. 1955 m. H. YOCOM AT TORNE V United States Patent" ELECTRON BEAM SYSTEM Rudolf Kompfner,, Far Hills, and Willis H. Yocom,

Chatham, N. .L, assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application june 10, 1955, Serial No. 514,423

12 Claims. (Cl.3153.5)

The present invention relates to an arrangement for focusing electrostatically a beam of charged particles and to apparatus which employs such a beam.

In the focusing of beams of charged particles, it is generally found desirable to provide focusing forces to overcome radial space charge forces and other effects which tend to make the beam diverge, Such focusing forces may be provided by either magnetic or electrostatic fields. The use of a magnetic field is often a disadvantage, especially when beams of high density and long paths are desired, since the equipment necessary to provide the desired magnetic flux is apt to be heavy and bulky. Accordingly, when feasible, it is preferable to employ electrostatic fields for focusing. Hitherto, electrostatic focusing has generally involved the use along the path of flow of an electric field which is time-constant but spatially-alternating in direction. To this end, there has been required an electrode system comprising an array of electrodes in which successive electrodes are maintained alternately at high and low electrostatic potentials. For many applications, focusing in this way is not completely satisfactory. In such an arrangement, the charged particles are passing alternately through regions of accelerating and retarding electrostatic fields, so that the average velocity of the particles is periodically undergoing significant change. This reduces the usefulness of the beam for many applications. Additionally, to establish and maintain the necessary potential differences between successive electrodes of the array irnposes requirements that complicate the design of apparatus utilizing such an electrostatically focused beam. Accordingly, there is a need in the electronic art for a simple arrangement for focusing an electron beam over an extended path of flow, which does not require either a magnetic field or spatially-alternating applied electric potentials.

To this end, the principal object of the present invention is to meet this need.

The present invention is based to a considerable extent on the discovery that associated with an electrode system comprising an array of positively biased conduetive elements spaced apart longitudinally there exists a pairof singular equipotential surfaces which are characterized in that a charged particle moving along either such equipotential surface with a correct velocity (which is a function of the geometry of the electrode system and the applied potentials) will remain substantially on that equipotential surface balanced between electrostatic and centrifugal forces. In particular, it is found that each of these singular equipotential surfaces forms a sinuous path Winding inand out between successive elements of the array in a manner suggestive of a skier tslaloming between successive flag markers. For this reason, it is thought appropriately descriptive to characterize the focusing technique of the invention as slam focusing.

The principles of the invention find particular appli- 2,857,548 P tented Oct. 21, 1958 cationfalthough not limited thereto, traveling wave tubes. In su'clr'tubes, it is desired to project an. electron beam closely past a {slow wave interaction circuit on which is propagating an electromagnetic wave in a man ner that the waveand beam interactto provide ampli fication of the wave; 'There are many. forms of interaction circuits known to be suitable, for goodinteraction which are ehara'cterized by a linear array of transversely extendingelementsI Such interaction. circuits are readily adapted for establishing a singularequipotential surface therealong which will serveas'a trajectory for an e c h' e '1 131 V The "e will be described as illustrative embodiments of the principleso'f theiijvention'several traveling wave tubes, in each of whieh the slow wave interaction circuit is adapted to serve asthe array of elements forming the basic portion of an electrode system used for. establishing a singular equipotential'surface. In combination with such an electrode systenn there is provided an electron gun arrangementflwhichfismade.to inject an electron beam on the singular equipotential surface for slaloming past the interaction ircu'it. T he invention will be better understood from the following mo're detailed description taken in. conjunction "tential distribution associated with, respectively, a linear arraiy Qflina a s nd a lin ar a 9 WiI lemen s ia p' s t y t sp ct I a b it. Qf b u din p a e. f Be 2 shows nf h m cf r ah el st on beam y tem in accoi'darice with theinvention; V

Figs. 3, 4' and 5 Show iltfirnative arrangements, for injecting'an ele'c'tron'beam into an electron beam system in accordanc 'ewith'the invention; and. e 7

Figs? 6? 7 3115.8 Q L n hrm oft av in wa e u hm ih electron a s tems n a co dance Wi he y fi id i l T r n w more pec cal vio the d awi in lfig. the're is shown in two, dimensions the equipotena r q or g t pot nt a d st i ut o ass ci ted h' plurality p t vefli cha 1.0 xte d n a z ae dp d apar tahc va for formin a q g fd hl r y' s e i fi sih the dir ction. I c n. be 9 ha yr n 2 5 t e potent l 1s given by whet? a i lz Pqt a al with re pect to an a b trary reference potent; (which for reasons to become apparent 1 4" "2h Y we w re i It is further charaeteristie that such singular equipotential' surfaces extend longitudinally winding sinuou sly between successive line charges" infthe manner suggestive ofja 'slalom pattern'as shown gra'phigally in Fig. 1A. These' two singular equip otential surfaces have a mirror symmetry with the'plane of the line charges 10 as the reflection plane. the {drawing there is also indicated the i'elative potentials bflvariohs other equipotential surfaces with respeetjto the potential V, characterizing the singular equipotentijalisurflacesg l l It is inaficordarice withthe invention to employ an electrode system' which provides a similar pattern of singular'equipotential surfaces. In Fig. 1B there is shown in two dimensions an illustrative electrode system 15. Taking the role of the positive line charges are the 'posi tively charged conductive -lements 16, typically wires extending in the-z directiomarranged in a linear array in which the successive elementsl-arespaced apart the distance a in the ac direction. The outer surface ofeach of these elements advantageously.coincides Wifl'l one of the closed equipotential surfaces of the plot shown in Fig. 1A and the potential applied to each such element should correspond to thepotential of theequipotential surface chosen.. Additionally, spaced in the y direction on opposite sides of the linear array of elements cxtend conductive members 17 and 18. The surface of each of thesemembers facing the linear array advantageously also a shape which substantially coincides with the equipotential surface of the plot shown-in Fig, 1B. In particular, it is advantageous to operatethe conductive bounding plates at "the'potential of the source from which the electrons originate. Thiscorresponds to the zero equipotential surface of Fig 1A which is substantially planar. Ordimadly, in the electrode systems of primary interest the members 17 and 18 willhe suificiently separated from the linear array-that they can be planar plates to conform to the substantially planar equipotential surfaces, shown in'Fig. 1A, characteristic of regions appreciably separated in they direction from the plane of the line charges.

By means not shown, each of the elements 16 of the array is maintained'at the same positive potential with respect to that of the boundary members 17 and 18. The potential of the singular equipotential surfaces may be chosen arbitrarily to have any desired value. Equation 1 then is solved with the parameters of the desired geom- V,,, the potential corresponding to that the singular equi-.

potential path with respect to the cathode fromwhich by acceleration from rest through a potential difierence the electrons in the beam to be focused originate.

etry used to fixs'ome of the boundary conditions and in this way there is determined the potentials which need to be applied to the elements It? and thebounding members 17 and 18 in order, to achieve singular equipotential surfaces of the kind desired. There then results the potential distribution in the interspace. between plates 17 and 18 V of the kind shown by the broken lines, which is identical with that shown in Fig. 1A. There are available as the boundary conditions which serve as possible variables the size of the conductive elements 16 (which incidentally fixes their optimum. shape), the spacingof the conductive members, 17 and 18f(which also fixes their optimum configuration) and th'potentialsTto-be" applied to the elements'16 and members 17'and18 A 1. I

in accordance with theinveiitiom it has beenfound thatif an electron is injectedgfor travel on either of the two singular equipotential 'surfaces'll and 12.with'a velocity v which is given by i where e and m are, respectively, thelchargeand mass of an electron, the electrostatic and centrifugal forces acting on the electron moving longitudinally willalways be equal and oppositealong such singular e'quipotential surface. Accordingly, an electron injected on one, of'these singular equipotential surfaces .with a velocity v inthe direction of the singular equipotential line will follow a trajectory defined by the singular equipotential surface. It will be convenient to describe a velocity which results in stable flow substantially alonga ingulq qui otential surface-as acorrect-velocity; Accordingly,;a sheet beam of electrons properly injected'ca'n be focused; for travel substantially along a singular, equipotential surface past an array of conductive elementsg It is to be noted that Equation 3 will be automatically satisfied if X' -is made the potential difference through which an election is accelerated from rest before itis injected on thesingular equipotential path, sincethe added velocity imparted to an electron in passing through a potential diiference equal to V is as given by Equation 3; J means thata correctvelocity willbe assured ifthe electronis accelerated to a velocitywhichicorresponds to the velocity imparted An examination of Equation 1 and the. number of variables whose value may be readily controlled shows:

that there are available more parameters than necessary..

arrangement. In particular, it is feasible to eliminate the conductive plates, which corresponds to an infinite separation of the plates from the array, or to have the separations of the individual plates from the linear array The latter arrangement necessitates a dif-f Alternatively, it is feasible to fix at a particular desired value the potential," the spacing, and size of the elements forming the array and thereafter to adjust the value of the potential of the plates and the plate spacing so that the singular equipo- It is also feas. ible to vary two or more of the parameters with travel in the direction of flow as shown in the arrangement of Fig. :5

be different. I ferent potential on the two plates.

tential path will have a desired potential.

to be discussed below without appreciable cumulative elements for forming a bounding equipotential surface approximating one of those shown in Fig. 1A. Finally,

even the potentials and sizes of the elements of the array may be varied if each elementsis made to coincide in.

position and potential with a closed. surfaceof Fig. 1A. Additionally, it can be shown that the velocity with P which the electrons are injected on the singular equipotential path may vary several percent without appreciable: In particular, it is found thatv the flow along the desired path is stable so that aminor ill eifects onthe focusing.

perturbance of the flow merely results in a trajectory which oscillates about the singular equipotential-path.

In Fig. 2 there is shown in perspective an electronbeam system 20in accordance with the'invention. An array of parallel wires 21, each of which extends in a direction transverse to the desired directionof flow which are spaced apart a uniform distance in the longitudinal direction' extends along the interspace between conductive plates 22 and 23. Each of' the wires is maintained at a positive potential with respect to the pair of plates. between an adjacent pair of wires is positioned an electron source which comprises. a filamentary cathode 24 extendingparallel to the individual elements .of .the array surrounded-by an anode 25. The anode 25 is positioned in the region of the intersection of the singul'aneqmpotential surfaces associated with the array of Wires, and.

maintained at a potential V which. corresponds to that characterizing the singular equipotential surfaces. Asa

consequencqthe eifect of the presence of the electron: gun is minimized with regard to any disturbance of the a potential distribution in the interspace, part cularly the singular equipotential surface. Additionally, the differ ence in potential between the cathode 24 and anode 25. j

is made equal to the potential V The anode 25 .is a cylindrical element which is slit for exit of the electrons therepast at a region which corresponds to a slngular' equipotential. Accordingly, the electrons passing from the cathode beyond the anode are formed into a sheet beam and'are injected on the desired singular equipoten-, tial surface shown bythe broken line at a correct velocltyu so that the beam has a trajectory which substantially corresponds to the singular equipotential surface. In a specific design of an arrangement of this kind, 32 wires each of 60 mils diameter, with a center ,to center spacmg of mils formed the linear array of elements positioned V midway between two conductive plates 300.mils apart.

Typical operating parameters included a'wire potential 7 Intermediate of: -I-3 90-vo1ts; an accelerating .anodeipot entiali of +2302. volts,---a:plate potential Of-s-lSO-Voltsg'f and a currentrof the beam has completed its necessaryrole in the application intended. For example, in traveling wavetube applications, it will generally be desirable to extend the electrode structure establishing the singularequipotential surface beyond the -end..of the-radio frequency signal circuit. This may be done by employing elementsin the linear array whicharenot. part of .the radio frequency circuit. In the arrangement shown, the target electrode includes a collectingelement 26 maintained at the=potential ofthe elements 21 'which' is partiallyjsurrounded by a collector housing ,27 maintained at the-potential 'of. the singular equipotential surface. By maintaining the housing. at this potentiah'disturbancyof the. potential distributionis minimized. The housingis slit at a point corresponding to .the interception ofthe singular equipotential surface which serves'as the path. of electron flow for entrance therethrough of the electrons to the collecting element. The collecting. element'is maintained at the higher potential to minimize giving rise to the emission of secondary electrons. Alternatively, the electrons .may be collected advantageously in' the manner described the copen'dihg application SeriaFNo; l4,42l,filed June 10, 1955, by]. S. Cook, R; Kompfnerand wjHfiYocom.

Iii-Fig: 3th'ere is' shown in' greateridetail a'form-of ele'ctrongun 30 suitable fdrnse as the "electron'source injth c'practice of- 'the invent-ion whenthe interelecuode' distances'rwhich are tolerable" are relatively large: A filamentary "heater wire- 3Pextendscoaxially through-a cylindrical cathode housing 32," a portion of whose outer" .surface'is made electron-emissive to serve "as thecathode.

The cathode" coating advantageously is positioned 'inthe electronstherepast. Lips are provided at the slit; for- Partially surrounding 'the 'beam forming beam shaping. element' 33is the acceleratingganode' '34," also appropriately 'apertured for'passage of the electrons therepast for injection on a singular equipotential surface! As before, the electrons are accelerated *by--a-=potential' V ,5 before m je'ction for travel alongthe singular equipotentialpath so as to have a correct velocity? The schemes already described for"injecting;the elec-" trons for flow along'a singular equipotential" have both involved"injectioninternally' from a region "included with the interspaceenclosed-by'the-bounding plates. For someapplications, it may beprefei'able toinject electrons from-a region external to suclr-an -interspacee Inpatticular, in applicationswh'ere the-linear array 'is' to serve as-a radio frequency;wave-propagating structure; it may in" some cases be desirable -to isolate the path for the radio frequency wave as much as possible from 'the electron gun. i b p In Fig. 4; there is shown" in: a-- longitudinal sectionalview analternative arrangement *40-for injectingelectrons with-a correct velocity on a singular equipotentialline.- In this case, an electron gun:comprising aL-cathode 41;,andn a beam -forming electrode: il-and accelerating anodeai43z are positioned external "to the interspace :anduprovide anelectron beam which? is projected into: the: 1. interspacew 6; theziregion of .therlinearrarray of' wires-.46; Positionedrasly aicontinuation of-& the linean; array, of 'wires..-46, is' ;a deflecting element 47 which is maintained at a potential. negative withrespect 'to the potential of the remaining wires 46 of, the array and is-usedqto deflect the "beam", injected into the interspace. The geometry ofthe. arrangcment and thevarious potentials applied to the-gun and deflecting element are adjusted so that electrons are: injectedwith a correct velocity .on-a singular equipoten tial surface 48 with the trajectory shown. It is feasible. to inject an electron beam .in an analogous manner. through an aperture .in plate 49 on to the other singular. equipotential surface.-

For many applications, it will be importanttowork. with beams whose transverse dimension inthe :-direction of plate separation is small but which have ratherhigh densities. Tothis end, it is desirable to employ an electron gun of relatively -large dimensions but toconverge, the electrons emitted therefrom into a-sheet beam of the. desired narrow transverse dimension. In-Fig. 5. lthere is. shown as5alongitudinal.sectionalyiew an electron gun. arrangement 50 suitable for such purposes. It characa terized'by'the inclusion of a tapered section in which the dimensions'of the focusing structure are gradually ta=. pered down from large initial sizes to smaller sizes more suited for the intended application. As shown,the spacing in the x direction between successive wires 51 forming longitudinal array is gradually decreased; together with the-radii of successive wires. Concurrently, the spacing in the u direction of the conductive bounding; plates 52 and 53 from thearray is gradually decreased... By making the rate of change with .distance of the,- various dimensions small compared with the distance an electronbeam properly injected will still substantially; follow longitudinally a singular 'equipotential. surface wtihout-anappreciableloss ofelectrons. The electron: beam may be injected by any of the arrangements pre viously described. By way of illustration, the external injection arrangement described in conjunction withv Fig. 4 is being shown inFig. 5 in which electrons emitted. from a cathode 41-positioned external to the interspace: between the bounding plates are formed into a beam. which is projected into the interspace through-ran aperture in one of the plates 52 and deflected with a--correctvelocity by a deflecting electrode 47 into the singular equipotential path.

Still another arrangement suitable for injecting an electron beam with a correct 'velocityon a singular equipotential surface is described in copending applicationr. Serial No. 514,421, filed June 10, 1955, by 1. 5. Cook, R. Kompfner and W. H. Yocom.

It will be convenient in. the traveling wave tubesto be discussed hereinafter whichutilize an electron .beam sys-' tem of the kind described as in accordance with theinvention to show only schematically the electron gunused to 1 inject the electron beam along a singular equipotential; surface. It is to be understood that any suitable. gun," of h which those described hereinare illustrative, may, be used.

In Fig; 6 there is shown a perspective viewof a: traveling wave tube fill whichincorporates the. focusing, principles of the. invention for providing electron. flow suitable for interaction with a signal-wave. Within-an evacuated .envelope. 61,- which typically may be of glass v thereis positioned a wave guiding circuit comprisinga zig-zag transmission'line 62. Such aline comprises a. conductor-which is folded back and forth a pluralitvof times, substantially in a single plane, successiveefoldsii being spaced apart in a. longitudinal direction; Theline: is shown schematically supported at its folded, ends.

It is known that a folded line of this kindfiS:Sllit6d-' for use as an interaction circuit for propagating; arrielec-z. tromagnetic wave. which hasaalongitudinal componentiofr through amopening 44'iin onemf the plates 45 bounding. ielectricafield-z having. ;a:: phase velocity; a zfractiongofi' the t speed of light so that'this velocity can be readily matched to the velocity-of an electron beam in the manner typical of traveling wave typeinteraction.

It is'evident that such a line provides in the longitudinal direction along the intermediate portion of the folds a succession of parallel conductive elements such as 62A, 62B and 62C spaced apart in a linear longitudinal array. Moreover, by positioning conductive plates 63 and 64 on opposite sides of the transmission line, there results an electrode structure which resembles that shown in Fig. 113.

By suitable lead-in connections from a D. C. voltage source the line may be maintained at a suitable positive potential with respect to the conductivemembers, so that a potential distribution characterized by a pair of singular equipotential surfaces of the kind displayed in Fig. 1A may be realized along the line.

An electron beam in sheet form i injected along one of such surfaces for travel past the transmission line by an electron gun 65 shown schematically positioned at one end of the line. By the techniques previously described, the electron beam is injected on one of the singular equipotential surfaces with a correct velocity. It is convenient for achieving an arrangement like that shown in Fig. 2 to position an auxiliary electrode 66 beyond each end of the line to serve as extensions of the linear array ofelements extending beyond the electron gun to minimize abrupt discontinuities in the potential distribution beyond the ends of the line.

For traveling wave operation, the longitudinal electron beam velocity must be matched to the phase velocity of a longitudinal electric field component of the traveling wave. The phase velocity of the wave can beadjusted by the geometry of the line in a maner known to workers in the art. The desired beam velocity can be'realized by appropriately fixing the potential of the singular equipotential surface. Such potential may be realized by proper choice of boundary conditions. The boundary conditions most easily adjusted are the D. C. potential of the line and the spacing of the conductive plates. It is generally convenient to operate the plates at the same potential as the cathode of the electron gun. To aid in confining the edges of the sheet beam, it is advantageous to include longitudinal projections 63A, 63B and 64A, 64B which extend from each plate 63 and 64 towards the linear array of elements at points slightly wider than the edges of the beam. Moreover, it is also feasible to employ two sheet beams injecting one on each of the two singular equipotential surfaces in-the manner previously described when high powers are important.

A traveling wave tube of the kind described may be employed in the forward'wave mode as an amplifier or in the backward-"wave mode as an amplifier or oscillator. For use as a'fo rward wave amplifier, the input signal is applied from a signal source to the transmission line forming the circuit at the upstream or electron source end and the output wave is abstracted at the downstream or collector end for use by the load. Connection to the circuit can be made simply by connecting the end of the circuit to the inner conductor of a coaxial line. Additionally, for use as a forward wave amplifier, it is advantageous to insert loss along the circuit to minimize the eifect of reflections in causing instability.

For use as a backward wave oscillator, the beam velocity is matched to the phase velocity of a negative space harmonic of a wave traveling along upstream the circuit in the manner characteristic of backward wave operation. Additionally, the beam current is made sufficiently high that oscillations are set up. The output oscillatory wave is abstracted for use by the external load by a connection to the upstream or gun end of the wave. Additionally, the downstream end of the circuit is terminated to be sub stantially reflectionless, as by the insertion of a section in which resistive lossy material is introduced gradually. The frequency of the oscillations may be varied readily the potentials of the electrode system are varied prop'ortionately. In particular, by supplying each of the potentials necessary for stable flow to the electrode system' by tapping at various appropriate points a potentiometer which is connected across a suitable voltage source,-variations made in the voltage of the source will result in proportionate variations in the potentials at the various taps in a way that will maintain undisturbed the various 'relations necessary for stable flow between the potentials of the electrode system.

For use as a backward wave amplifier, the beam velocity is again matched to the phase velocity of a negative space harmonic traveling upstream along the circuit.

The input signal is applied to the circuit at its downstream end and the amplified output abstracted at the upstream 'end. Caution must be taken to insure that the beam current is insuflicient to set up backward wave oscillations and the loss along the line is minimized.

Alternatively, the backward wave oscillator may be of the kind described in copending application Serial No.

- 392,946, filed November 18, 1953, by H. Heffner and R;

Kompfner in which a backward wave circuit sets up oscillatory space charge waves on the beam which are ab-f stracted by a forward wave circuit positioned downstream of the backward wave circuit.

In Fig. 7 there is shown in perspective a traveling wave tube which employs an interdigital transmission line as the wave interaction circuit.

above. Within an evacuated envelope 71, suitably posi Such a tube can be operated as a forwardwave amplifier, backward wave amplifier and backward Wave oscillator in accordancewith principles known to workers in the art describedv tioned is an interdigital transmission line of the kind 1 known to workers in the art and described in detail in U. S. Patent 2,823,332, issued February 11, 1958, to R. C. Fletcher. This line comprises a linear array of conductive finger elements 72, successive finger elements extending from opposite conductive end support members 73, 74 for forming an interdigital array which serves to provide a zigzagging path for the traveling wave in a manner resembling the wave path provided by the zigzag line of the tube shown in Fig. 6. Positioned on opposite faces of the interdigital line are the top and bottom Q conductive plates 75, 76. By insulating the interdigital f line from the plates 75, 76 and establishing a potential difierence therebetween, there is effectively achieved .a r.

structure which functionally resembles that of Fig. 1B

and, accordingly, establishes a pair of singular equipotential surfaces which serve as stable trajectories for electrons injected properly thereon. In the manner described above, sheet beams of electrons may be injected from an electron source 77 on each of these singular equipotential surfaces with a correct velocity'for substantially following these singular equipotential surfaces. The potential of the singular equipotential surface is,

chosen so that the longitudinal velocity of electron flow will be matched with the phase velocity of the longi-ltudinal electric field signal component with which interaction is desired. Coupling connections may be pro-- vided to the ends of the interaction circuit in the manner 1 known to workers in the art.

In Fig. 8 there is shown in perspective a traveling wave tube 80 of the kind which employs a' helix as the interaction circuit and in which the principles of the invention are incorporated. Within an evacuated envelope 81 there a is supported a helix 82 which is to serve as the interaction eircuit. To adapt better the helix for use as a focusing electrode in accordance with the invention, at

least one portion 82A of the helix is flattened to provide 1 a substantially coplanar linear array. of spaced conductive fi am nts which can aerveas a linear array of elements of the kind shown in Fig. 1B. Positioned on opposite .sides f this array of elements are a pair :of conductive plates 34 whieh'ser-ve as 111E bounding plates shown in the arrangement of Fig.'lB. The helix'is insulated from the plates and maintained at a suitable positive potential therewith whereby there are set up a pair of singular equipotential surfaces which zexteud past this flattened portion of the helix. A sheet beam of electrons is injected on this surface with a correct velocity from a suitable electron source 85. As in the ,tubes described earlier, the potential of the singular equipotential surfaces is chosen so that an electron velocity correct for focusing is also suited for interaction between the beam and a signal wave propagating along the helix. The signal wave may be applied to and abstracted from opposite ends of $3 circuit by coupling techniques known to workers in e art.

It will also be apparent to workers in the art that the principles employed in the tube shown in Fig. 8 may be extended to the projection of an annular electron beam past a conventionalhelix, i. e., one whose winding is smooth and whose cross section is substantially circular. In such a case, inner and outer cylindrical electrodes coaxial with the helix would serve as the conductive bounding members and the successive turns of the helix form a longitudinal array of parallel conductive elements. There will then be set up a pair of helicoidal singular equipotential surfaces, each of which winds sinuously past successive turns of the helix. Each of these surfaces will be closed and a cross section thereof taken normal to the helix axis will be substantially circular.

The focusing principles of the invention are by no means limited to applications in traveling wave tubes. Typical of other electronic devices which may be devised consistent wtih the principles of the invention are switching tubes and storage tubes of the kind described in copending application Serial No. 514,424, filed June 10, 1955, by R. Kompfner.

Accordingly, it is to be understood that the specific embodiments described in detail are merely illustrative of the general principles of the invention. Various other arrangements may be devised by one skilled in the art without departing from the spirit and scope of the invention. For example, although in the arrangements depicted the elements have always been arranged in a linear array, it is feasible to arrange them in a curved array so long as appropriate changes are made in the boundary conditions associated therewith.

What is claimed is:

1. In an electron discharge device, in combination, a source of charged particles, a plurality of conductive elements spaced apart in a longitudinal array, the elements being maintained at a potential difference with respect to said source for-forming a singular equipotential path which extends in a longitudinal direction wnding sinuously past the elements, and means for njecting particles from said source into said singular equipotential path at a velocity for following substantially along said singular equipotential path.

2. In an electron discharge device, in combination, a plurality of parallel conductive elements spaced apart in a longitudinal array, each element being maintained at a positive potential with respect to a reference level for forming a singular equipotential path which extends in a longitudinal direction winding past the elements, means at the reference potential level providing an electron beam, and means maintained at the potential of the singular equipotential surface for accelerating the electrons and injecting the electrons into the singular equipotential path with a velocity for following said path.

3. In an electron discharge device, in combination, a spaced pair of conductive means bounding the interspace therebetween, a plurality of conductive elements spaced apart in an array which extends longitudinally in said 10 interspace, means for biasing the elements with respect to said bounding means and establishing a pair of singular equipotential surfaces each of which extends longitudinally along said interspacewinding sinuously past the elements, and means for injecting electrons on to at least one of said equipotential surfaces at a velocity for following substantially longitudinally along said one surface.

4.- In an electron discharge device, in combination, a spaced pair of conductive plates, a plurality of conductive elements spaced apart and extending transversely for forming a longitudinal array in the interspace between the pair of plates, means for biasing the elements positive with respect to said lates for establishi g a Pai of sin ular equipotential surfaces, each of which extends longitudinally winding sinuously past the elements in the interspace, and means for injecting electrons into at least one of said singular equipotential surfaces at a velocity for following substantially longitudinally along said surface.

5. In an electron discharge device, in combination, a spaced pair of conductive plates, a plurality of spaced conductive elements forming a linear array which extends longitudinally in the interspace between the pair of plates equidistant from the two plates, means for biasing the elements of the array positive with respect to said plates and establishing a pair of singular equipotential surfaces which extend longitudinally in the interspace winding sinuously past the elements, means forming an electron beam, and means maintained at the potential of the singular equipotential surface for accelerating the electrons in said beam and injecting the electron beam onto one of said singular equipotential surfaces at a velocity for following substantially along said singular equipotential surface.

6. In an electron discharge device, in combination, a plurality of parallel conductive elements spaced apart in a longitudinal array, each of the elements being maintained at a positive potential for establishing a pair of singular equipotential paths, each of which extends longitudinally winding past the elements, and means positioned substantially at an intersection of the pair of singular equipotential surfaces for injecting electrons into one of said singular equipotential paths for travel substantially therealong at the correct velocity.

7. In an electron discharge device, in combination, a plurality of parallel conductive elements spaced apart in a longitudinal array, one of a pair of conductive bounding means on opposite sides of the longitudinal array, means for maintaining the elements of the array at a positive potential with respect to said conductive bounding means for forming a pair of singular equipotential paths which extend longitudinally in the interspace between the conductive bounding means, and means positioned external to said interspace for forming an electron beam which is injected into said interspace at a velocity for substantially along one of said singular equipotential surfaces.

8. In an electron discharge device, in combination, a transmission line for propagating an electromagnetic wave, portions of the transmission line forming an array of conductive elements, an electron source, means for biasing said portions positive to said electron source for forming a pair of single equipotential surfaces extending past said portions, and means for injecting electrons from said source into one of said single equipotential surfaces with a velocity to follow substantially along said single equipotential surface.

9. In an electron discharge device, in combination, an electron source providing a sheet beam of electrons, a

transmission line which comprises a conductor which is folded back and forth to form a zig-zagging wave path, portions of the transmission line forming a linear array of parallel conductive elements along the beam path, means for biasing said portions positive to said electron source for forming a pair of singular equipotential surfaces winding between successive portions of the linear 1 1 array, and means injecting .saidsheet beam onto one of said singular equipotential surfaces with a .rvelocityt for following substantiallyalong said surface. 1

10. An electron'discharge'device comprising means for defining an electric field characterized by a plurality of equipotential surfaces, said means including a longitudinally extending array of spaced electrodes, conducting means on each side of said array and spaced therefrom,

-a velocity "to cause said electrons to travel substantially along said one. equipotential surface.

11. Anqelectron discharge device as'claimed inclaini :10 wherein said conducting tmeans are maintained at a 5 potential negative with vrespect to the electrodesinsaid array. a 12. An electron discharge device as claimed in'claim 10 wherein said array constitutes a slow wave circuit.

7 References Cited in the file of zthis patent UNITED STATES PATENTS? Pierce Jan. 14, 1 947 Lee a Aug,' 24 1954 Warnecke et. a1. Aug. 31,= 19 54 

